Electrocardiogram Interpretation – How to Determine the Heart Rate

Estimating the heart rate from an electrocardiogram (EKG) is one of the fundamental steps of EKG interpretation, playing an important role in the diagnosis and treatment of heart disease. With the EKG as a diagnostic tool, the healthcare provider can promptly detect both overt and subclinical deviations from the normal rate and rhythm.

Normally, the sinoatrial (SA) node controls the rate of successive cardiac cycles. The action potential or cardiac impulse is generated in the SA node found in the atria, and is then conducted rapidly into the ventricles via the atrioventricular (AV) nodal/His bundle system, the bundle branches, and the Purkinje fibers to produce ventricular contraction.

The normal human heart beats at a rate of 60 to 100 times per minute. A heart rate exceeding 200 beats per minute is not uncommon during heavy exercise. Physically fit individuals, especially endurance athletes can have resting heart rates as low as 40 beats per minute. There are different methods of determining the heart rate on an EKG tracing.

The quick method (used when rhythm is regular):

How to use this method:

Look for an R wave coinciding with a bold line. Then, count the number of large squares between this R wave and the next R wave. Determine the rate using the table below.

1 large square

300/1 =

300 per minute

2 large squares

300/2 =

150 per minute

3 large squares

300/3 =

100 per minute

4 large squares

300/4 =

75 per minute

5 large squares

300/5 =

60 per minute

6 large squares

300/6 =

50 per minute

It is recommended to memorize these numbers by dividing them into two groups:

300, 150, 100

75, 60, 50

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When the second R wave does not coincide with a bold line, apply the method described above using the bold line nearest to the second R wave as the boundary of the last square to be counted. However, remember these points:

If the second R wave is to the left of the nearest bold line, the rate is higher than the quotient obtained

If the second R wave is to the right of the nearest bold line, the rate is lower than the quotient obtained

How to determine the heart rate when it is lower than 50 beats per minute (regular rhythm):

Count the number of large squares between two successive R waves.

Divide 300 by the number of large squares obtained in #1.

The quotient is the heart rate (cardiac cycles per minute or heart beat per minute)

How to determine the heart rate when the rhythm is irregular:

Method A:

In a standard EKG tracing, the electrical activity of the heart is measured for 10 continuous seconds. Count the number of QRS complexes in one continuous lead where cardiac activity was monitored for the whole 10 seconds or in contiguous leads encompassing 10 seconds of cardiac activity. Multiply this number by 6 (10 x 6 = 60 seconds = 1 minute) to obtain the number of heartbeats per minute or the average heart rate.

Method B:

Count the number of QRS-complexes within 30 large squares. This is equivalent to 30 x 200 ms or 6 seconds of cardiac activity. Multiply this number by 10 (6 x 10 = 60 seconds = 1 minute) to get the number of heartbeats per minute or the average heart rate.

The methods illustrated above provide a simplified approach to determining the heart rate through EKG interpretation. Whichever method one prefers to use, it is important to always correlate findings with the overall clinical picture. For more in-depth discussions and interactive exercises to help you develop your EKG reading skills, please visit Medical-ELearning.com.